Thickness indicates how warm or cold the atmosphere is, the lower the value the colder it is. In winter a value of 528dam or less often indicates it will be cold enough for snow.
850 hPa temp is the temperature at the 850 hPa level. It can be good indicator for cold spells in the winter, with values of -10C or less giving the potential for heavy snow showers to develop.
Total cloud cover is the forecast for the percentage of the sky which will be covered by cloud. Layers of cloud form at different levels and this value tries approximates the combined effect.
Snow depth is experimental and tries to provide an indication of the depth snow accumulations since the start of the forecast period.
Freezing level is the forecast for the level in metres at which the temperatures will be 0C. Local factors and low level inversions mean that often it can be below 0C even if the freezing level is much higher than ground level.